One of France's most eminent art experts has criticised the Louvre Museum's cleaning of a 500-year-old Leonardo da Vinci masterpiece, The Virgin and Child With Saint Anne.

Ségolène Bergeon Langle, former director of conservation for France's national museums, accuses the team involved with the restoration of removing details of Leonardo's original work by mistaking it for repaints by later hands.

She criticises the removal of Leonardo's own glaze on the infant's body. A dozen letters she wrote "opposing the cleaning" and warning of damage to one of western art's jewels "remained unanswered", she said.

Two of the experts on the international committee that advised on cleaning were specialists from the National Gallery in London, Larry Keith and Luke Syson.

Bergeon Langle, an authority on art restoration, resigned from the committee, along with Jean-Pierre Cuzin, the Louvre's former head of paintings, last December. At the time neither would discuss their departure. Bergeon Langle said only: "My reasons I am reserving for a meeting with the president-director of the Louvre."

But this week, she gave an interview to a prestigious French publication, Le Journal des Arts.

In January 2011, she said, the committee agreed "a gentle cleaning" of late varnishes and the removal of stains on the Virgin's cloak: "Yet, between July and October, a more pronounced clean was called 'necessary', which I objected to. I was then faced with opposition to my position, which is technical, not aesthetic." Her letters to the committee asking for details on the cleaning and materials to be used for retouching went unanswered, she said. "I had to resign."

She believes the decision-makers behind the restoration were not cautious enough. "The Virgin's face is less modelled now. The cleaning should never have gone so far … The whitened layer on Christ Child's body has been mistakenly understood as a late varnish gone mouldy." She believes it was an irreversibly [original] altered glaze: "I recommended preservation, but nobody would listen."

Considering that experts rarely speak out, her decision to go public is damning. It confirms the view of critics that the painting is left too bright and is robbed of the Renaissance master's subtlety.

As the Guardian reported last year, a Louvre source revealed that Keith and Syson were particularly keen on this restoration.

Michel Favre-Félix, president of the Association for the Respect and Integrity of Artistic Heritage in Paris, was among those alarmed by the cleaning procedures. He is now collating a dossier of evidence, challenging the restoration on "points of gross misconduct".

He said: "We are calling for the establishment of a scientific ethics committee, independent from the museums and the restoration teams, just as there is for medicine. It should re-examine the whole Saint Anne file."

Michael Daley, director of ArtWatch UK, the restoration watchdog, said he was shocked to learn that Bergeon Langle's warnings had been ignored: "It suggests either that describing what they planned would be dangerous and embarrassing or that they weren't clear what they intended to do. Either way, it's unacceptable."

The criticisms resonated with another member of the Louvre's advisory committee, Jacques Franck, consulting expert to the Armand Hammer Centre for Leonardo studies at the University of California, Los Angeles. He told the Guardian: "Restorations likely to generate such disapproval from leading experts should never be undertaken … Bergeon Langle is unquestionably France's best authority."

Asked to comment on whether other Leonardo paintings in the Louvre should be cleaned, Bergeon Langle gasped in horror: "Just do not do it!"

The Louvre and the National Gallery declined to comment.

• This article was amended on 30 April 2012. The original said that Ségolène Bergeon Langle criticised the retouching on the landscape in the painting and that she believed the restorers were not cautious enough. These points have been corrected.